A gas-insulated three-phase cable made up of lengths that are about 100 meters long has already been described, in particular in French patent application No. 93 15 355. Each length is made by welding together elements that are themselves about 10 meters long. Each of these elements comprises:
a cylindrical steel case that withstands the internal pressure of the insulating gas, which is preferably nitrogen under a pressure of about 10 bars; PA1 a cylindrical aluminum case, coaxial with the steel case and serving as a magnetic screen between the conductors and the steel case; and PA1 three conductors disposed inside the aluminum case and disposed parallel to one another and to the axis of the cases. The conductors are held in place by insulating arms supported by metal arches.
At one of the ends of each length, there is placed a metal disk which is secured to the steel case. The disk is provided with three openings through which the ends of the conductors pass. The conductor ends are held in insulating cones fixed to the metal disk. A conductor in a given length is electrically connected by engaging the end of the conductor in a metal sleeve fixed to the end of the conductor of the adjacent length. The inside of the sleeve is provided with contact pieces, e.g. of the concertina type. This sliding contact makes it possible to continue passing current in spite of differential expansion between the conductors and the case of a length.
To be sure of providing sufficient insulation distance between the conductors and the metal disk which is at ground potential, it is necessary to reduce the section of the conductors where they pass through the openings and where they approach said zone, and to do so on both sides of the disk. In the event of the conductors carrying overload current, large attraction forces appear between the conductors, thereby generating vibration at twice the frequency of the current. Such vibration, accentuated by the smaller diameter of the conductors at their ends, gives rise to pitting of the contacts, thereby degrading them and producing harmful heating.